Why are my Garmin and Apple Watch calorie counts different?

I use an Apple Watch Ultra for daily activities and a Garmin Edge for biking. Apple Health is used to import all data to Athlytic that I use to track my "fitness". Now my problem: originally, I did not import my Garmin data into health and therefore also not to Athlytic. Instead, I recorded biking sessions both on Edge and Apple Watch, which was quite inconvenient. I also noticed that the calories Apple showed appeared significantly higher than what Garmin had for the same workout. I decided to start importing Garmin to Apple Health, and stopped tracking the biking workouts on the Apple Watch, assuming that this would get my Garmin data into Health and to replace the Apple Watch data. The result was that the workout minutes get recorded correctly and contribute to my Apple Fitness exercise minutes, but the calories burned did not add up. Looking at the data in Apple Health, it recorded the data from Garmin, but did not include them in the moving or total calories number. So I got curious: as I saw that my Apple Watch was recording calories for the same time periods in Health as well (I was wearing the watch, just not recording a workout), those calories were significantly lower now than the calories that Garmin recorded - presumably because of the longer time intervals that the Apple Watch uses to measure when not in a Workout mode. I manually deleted all the calorie recordings coming from the Apple Watch that were recorded during the time of the workout, and - my "active calories" increased! Once I did this, Apple Health was now counting my Garmin calories! As I have not found a better solution yet, this means for me now that I either have to turn off calorie tracking on my Apple Watch while doing a workout, or leaving the watch at home all together when riding my bike. Does anybody have a better solution for this? I can come up with a good explanation why Apple might have done this - avoiding double counting calories from two different sources for the same activity - but it seems odd to give then this preference to the Apple Watch count although it seems really far off counting active calories when it is not in workout mode.

[Re-Titled by Moderator]